The need for converting direct current electrical energy to alternating current electrical energy has existed for the greater part of this century due to the advantageous property of alternating current electricity in being able to be readily convertible to other voltages of alternating current by use of transformers. However, most electrical energy storage devices can only store direct current electricity. The prime example of this is, of course, the battery. Thus, if alternating current is desired, and an alternating current generating facility is unobtainable or inoperative, it is generally necessary to use electrical energy stored in a battery and to convert this direct current electricity into alternating current electricity by means of an inverter.
Over the years, several different techniques have been used for converting direct current electricity to alternating current electricity. Some of these devices are electro-mechanical and were commonly found on aircraft. Other inverters have used series resonant parallel filters to shape an electrically or mechanically chopped source of direct current into a sinusoidal voltage. However, the use of series resonant parallel filters generally require the additional use of sensing transformers, comparator circuits and duty cycle control drive circuits in order to sense the output of the series resonant parallel filter and thereby control the power switch circuitry that chops the direct current source of electricity. Such a feedback circuit is required in order to prevent excessive voltage variations on the output of the resonant filter. This particular method of converting direct current electricity to alternating current electricity is extremely expensive, due to the large number of electronic components necessary as well as the need for relatively large electrical components incorporated in the series resonant parallel filter. Furthermore, this series resonant filter generally requires individualized manufacture thereof due to the precise matching of components necessary for the filter to work properly.
Another method of converting direct current to alternating current is by generating a substantially sinusoidal electrical signal by sequentially switching direct current voltages, each voltage at a different level so that when switched in the proper sequence a step-like output signal is obtained that approximates a sinusoidal wave. By filtering this step-like signal, a relatively pure sinusoidal signal is obtainable.
These various methods mentioned all have significant drawbacks with respect to a low-cost, high-efficiency inverter that is capable of supplying external loads in the kilowatt range. The electro-mechanical conversions have the inherent problem of mechanical failure due to wear in the mechanical parts and also the disadvantage of moving parts. The series resonant parallel filter used in the second type of inverter is rather large and is expensive to manufacture. This type of inverter also requires a feedback circuit to regulate the output voltage from this filter. The cost for this type of inverter is several magnitudes greater than the cost of a comparable output inverter according to the present invention. The high frequency switching technique is also several magnitudes more expensive than the present invention and requires a significantly greater number of component parts.
Furthermore, the present invention incorporates overload and short-circuit protection circuitry without the need for fuses or circuit breakers. This circuitry responds so quickly to improper loading that catastrophic failure within the inverter is averted.
The present invention is especially well suited for emergency lighting used during power failures. The invention, while not having the precise voltage, frequency, and waveform purity found on some of the prior art inverters, is able to maintain a relatively constant frequency, constant voltage sinusoidal shaped output voltage within specifications for most devices operating on utility company grade 110 or 220 VAC electricity. Such minor variations in the output voltage, frequency, and waveform thus do not adversely affect most interconnected devices, and especially do not affect electrical lighting systems; and the resultant low cost and high efficiency of the present invention greatly outweighs and minor variations in these above parameters. The sine wave output also tends to reduce the radio frequency interference that is generated in square wave output type inverters.